A typical mortise and tenon joint can be seen in FIG. 1 and is used to secure two workpieces 30, 32 together. A mortise and tenon joint consists of a tongue 34 (the “tenon”) formed on one work piece 30 which fits into a corresponding shaped hole 36 (the “mortise”) formed in the second work piece 32. Typically the work pieces are timber. The size and shape of the tenon 34 is the same as that of the mortise 36 to provide a secure fit and thus a stable joint between the two work pieces 30, 32.
A sketch of a typical planer and thicknesser is shown in FIG. 2 and comprises a box like frame 4 comprising a horizontal rectangular upper table 14,16 and a base 6 connected to each other along their longer sides by two side walls 8. The horizontal upper table 14, 16 is located directly above the rectangular base 6. A first aperture 12 is formed by one of the shorter ends of the upper table 14, 16, by the base 6 and by one end of each of the two side walls 8. Similarly, a second aperture (not visible) is formed on the opposite side of the rectangular box frame by the other shorter end of the upper table 14, 16, the other end of the base 6, together with the other ends of each of the side walls 8. A passage way 22 connects the two apertures 12 to each other.
The upper table is constructed from two rectangular sections, a front section 14 and a rear section 16. The two sections 14, 16 of the upper table are constructed from single rectangular sheets of metal having smooth top surfaces. The top surface of the front section 14 is parallel to the top surface of the rear section 16. Both the top surfaces are smooth so that a work piece can be slid across their surfaces. The height of the front section 14 can be adjusted relative to the height of the rear section 16. The two sections 14, 16 are separated by a slot 18.
A horizontal lower table 20 is located movably within the passage way 22. The plane of the lower table 20 is parallel to that of the upper table 14, 16. The lower table 20 is constructed as single rectangular sheet of metal having a smooth top surface. The lower table 20 extends through the full length of the passage way 22 from the first aperture 12 to the second aperture. The width of the table 20 is slightly less than that the width of the passage way 22. The table 20 is mounted in such a manner that it can be moved vertically upwards or downwards by a height adjustment mechanism 190, the top surface of the table remaining horizontal at all times during this process.
A cutting drum 24 is rotatably mounted within the frame 4 in such a manner that its axis of rotation is perpendicular to the plane of the side walls 8 and parallel to the planes of the upper 14, 16 and lower 20 tables. The cutting drum 24 can be rotatably driven by an electric motor (not shown) mounted within the base 6.
A part of the periphery of the cutting drum 24 along its length extends through the slot 18 between the front 14 and rear 16 sections of the upper table.
A cutting blade is mounted within each of two grooves 28 of the cutting drum 24 which runs along the length of the cutting drum 24 in well known manner parallel to the axis of rotation. The cutting blades of the cutting drum 24 can be used to cut work pieces in well known manner which are either slid across the upper table 14, 16 in one direction or are slid across the lower table 20 in the other direction.
The cutting drum 24 is located so that, as the cutting drum 24 rotates, the maximum height of the cutting blade mounted within the cutting drum 24 through the slot 18 is the same as that of the height of the rear section 16 of the upper table, the height of the rear section being fixed.
Two drive rollers (not shown) are mounted on either side of the cutting drum 24 inside the frame 4 between the side walls 8 in such a manner that their axes of rotation are parallel to that of the cutting drum 24. The two drive rollers are rotatably driven by the same electric motor which is used to drive cutting drum 24. The function of the two drive rollers is to force any work pieces which are fed through the passageway 22 to slide across the lower table 20 and engage with the cutting blades as they pass below the axis of rotation of rotating cutting drum 24 at its lowest point in well known manner.
A planer and thicknesser can be used in two different modes of operation.
In the first mode of operation, a workpiece is slid across the upper table 14, 16 in order to remove the surface of the work piece which is adjacent to the smooth top surface of the upper table 14, 16. The height of the front section 14 of the upper table determines the amount of material which is to removed from the work piece. The height of the front section 14 is adjusted so that the cutting action of the rotating drum 24 removes the right thickness of material from the lower surface of the work piece. The cutting drum 24 is then rotatingly driven by the electric motor. Whilst the cutting drum 24 is rotating, the work piece is slid across the front section 14 of the upper table until it engages with the cutting blade of the cutting drum 24 as it rotates, which repeatedly passes through the slot 18 between the front 14 and rear 16 sections. It is then slid onto the rear section 16 of the upper table across the rotating cutting drum 24. As the work piece passes over the rotating blades of the cutting drum 24, the cutting blades remove material from the underside of the work piece.
In the second mode of operation, a work piece is slid across the smooth surface of the lower table 20 in order to remove the top surface of the work piece. The height of the lower table 20 within the passageway determines the amount of material which will be removed from the top surface of the work piece as it is passes through the passageway. The height of the lower table 20 is adjusted so that the cutting action of the rotating drum 24 removes the correct thickness of material from the top surface of the work piece. The cutting drum 24 is then rotatingly driven by the electric motor. Whilst the cutting drum 24 is rotating, the work piece is slid across the lower table 20, until the upper surface of the work piece engages with the rotating cutting blades of the cutting drum 24 as the cutting drum rotates. As a work piece passes under the cutting blades, the cutting blades remove material from the topside of the workiece. The two drive rollers, which are also being rotatingly driven by the electric motor force the work piece through the passageway 22.
UK patent application number 0404557.1 provides a description of one design of planer and thicknesser.
Most planer and thicknessers have provision for a cutting tool 40 to be attached to the end of the cutting drum 24. The most common type of cutting tool 40 is one for making mortises 36 for mortise and tenon joints. The cutting tool 40, when mounted on the end of the cutting drum 24, is co-axial with the drum 24 and projects from the side of the cutting drum 24 as shown in FIG. 2. When the cutting drum 24 is rotatingly driven by the motor, the cutting tool 40 also rotates.
Though the cutting tool 40 can be attached and removed as desired, it is preferable to be able to leave the cutting tool 40 attached thus saving the operator time. However, when the cutting drum 24 is rotated for any reason, the cutting tool 40 also rotates. This results in a hazard to the operator when the cutting tool 40 is not intended to be used, for example, when the planer and thicknesser is planing a work piece.